A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The size of structural details of devices that are manufactured with lithographic apparatus is continually decreasing. Accordingly, there has been a desire to use light (more generally radiation) of increasingly shorter wavelength into the UV and EUV range. Synchrotrons have been proposed as light sources, for example. Optical apparatus that use radiation of such short wavelengths typically use reflective mirrors instead of (refractive) lenses. Unfortunately, it has been found that these mirrors tend to degrade due to reactions at their surface, with chemicals that are induced by the radiation. Radiation of short wavelengths gives rise to a photoelectric effect, whereby electrons are liberated from the mirrors. These electrons, in turn, chemically activate gas molecules, like oxygen or water molecules, at or near the mirror surface and the activated molecules react with the surface.
In U.S. Pat. No. 6,533,952 a process called “mitigation” has been proposed to solve this problem. Similar techniques are described in WO 02/05347. “Mitigation” as used herein is the limitation of degradation of optical surfaces by the introduction of gases. Basically, when mitigation is used, gas molecules that would give rise to the contamination problem are added such that their effects balance.
A hydrocarbon is known to grow carbon on a mirror when illuminated with EUV, while for example oxygen is known to oxidize the top layer during exposure. A mixture of molecules of a first type (e.g hydrocarbons) and second type (e.g. H2O or O2) is introduced in the space near the surface of the mirror. The ratio between molecules of the first and second type in the mixture is selected such that permanent effects on the mirror are largely prevented. The pressure of the gases added to the lithography tool to this purpose is selected much higher than that of background gas molecules that are present, so that these cannot significantly affect the reaction balance.
This mitigation method works well when a pseudo-continuous EUV source like a synchroton is used. However, a synchroton is very costly and large. It would be preferred to use less expensive radiation sources like plasma sources. However, in contrast to a synchroton these are pulsed sources with a very low duty cycle. It has been found that the introduction of gases as such does not lead to mitigation when such a pulsed radiation source with a low duty cycle is used. On the contrary, introduction of a mitigation mixture even appears to increase degradation of the optical surfaces.